Magnetic media are widely used in various applications, particularly in the computer industry. A conventional longitudinal recording disk medium 1 used in computer-related applications is schematically depicted in FIG. 1 and comprises a non-magnetic metal substrate 10, typically of an aluminum (Al) alloy, such as an aluminum-magnesium (Al--Mg) alloy having sequentially deposited thereon a plating layer 11, such as of amorphous nickel-phosphorus (Ni--P), a polycrystalline underlayer 12, typically of chromium (Cr) or a Cr-based alloy, a magnetic layer 13, e.g., of a cobalt (Co)-based alloy, and a protective overcoat layer 14, typically containing carbon (C). The Co-based alloy magnetic layer 13 deposited by conventional techniques, e.g., sputtering, normally comprises polycrystallites epitaxially grown on the polycrystalline Cr or Cr-based alloy underlayer 12.
In operation of medium 1, the magnetic layer 13 can be locally magnetized by a write transducer, or write head, to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based on the bits of information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium layer 13, then the grains of the polycrystalline medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium can subsequently produce an electrical response in a read transducer, allowing the stored information to be read.
Thin film magnetic recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (CSS) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, allowing data to be recorded on and retrieved from the surface of the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk, and stopping.
It is considered desirable during reading and recording operations to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk to be positioned in close proximity, with an attendant increase in predictability and consistent behavior of the air bearing supporting the head during motion. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start-up and stopping phases of the cyclic sequence, thereby causing wear to the head and recording surfaces, eventually leading to what is referred to as "head crash". Thus, there are competing goals of reducing head/disk friction and minimizing transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk recording medium with a toughened recording surface to reduce head/disk friction by techniques generally known as "texturing". Conventional texturing techniques involve polishing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition thereon of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface of the underlying substrate is intended to be substantially replicated in the subsequently deposited layers.
A variety of techniques, including laser-based techniques, have been developed for texturing metal-based magnetic recording medium substrates, e.g., the Ni--P plated Al-based substrates described supra. Such substrates, however, exhibit low head impact resistance due to the low mechanical yield strength (e.g., as reflected by Young's Modulus values less than about 72 Gpa), thereby limiting their utility such that they are not particularly desirable for use in mobile computer data storage applications, such as lap-top computers. As compared to conventional, Ni--P plated, Al-based substrates, glass, glass-ceramic, ceramic, and metal-ceramic substrates having greater values of Young's modulus exhibit superior shock resistance. Accordingly, such "alternative" substrates are desirable candidates for use in data storage applications, particularly mobile computer applications. In addition to the requirement for good shock resistance, the "alternatives" type substrates are required to provide good vibration performance, especially when utilized in high rpm disk drives.
A number of advanced, high track per inch (TPI), low track misregistration (TMR), and non-repeatable run-out (NRRO) alternative substrates have been proposed for use in hard disk drive applications. However, none of the proposed alternative substrates has been utilized for the manufacture of practical disk drives, for the following reasons:
1. Poor lapability/grindability: in general, the glass, ceramic, and glass-ceramic and metal-ceramic composite materials contemplated for use as hard disk substrates are extremely difficult to lap or grind according to conventional techniques. More specifically, pure ceramic materials such as alumina (Al.sub.2 O.sub.3) are too hard to grind, and metal-ceramic composites (e.g., ceramic within a metal matrix) contain at least two non-uniform phases, i.e., a soft phase and a hard phase, which make the grinding process even more difficult. Moreover, the ultimate cost for grinding such substrates is significantly higher than that for conventional Ni--P plated, Al-based substrates.
2. Poor platability: due to the multi-phase nature and multi-crystal features of such alternative substrates, plating of a Ni--P seed layer for ensuring proper polycrystallinity of a Cr-based underlayer is necessary, as in the case of conventional Al-based substrates. However, the requirements for low TMR and high TPI require formation of Ni--P seed layers with defect-free surfaces after plating and/or polishing, with an attendant requirement for planarity which is higher than that required for conventional Al-based substrates. To date, none of the tested alternative substrates has evinced an ability to achieve a surface finish even approaching that of the conventional Ni--P plated, Al alloy substrates.
3. Plating of non-conductive disks: currently available non-conductive substrate candidates for use as disk-type magnetic recording media include glass, glass-ceramics, and ceramics, none of which provide the catalytically active surface which is requisite for electroless plating of the amorphous seed layer (typically of Ni--P) thereon prior to deposition of the polycrystalline underlayer (typically of Cr or a Cr-based alloy). As a consequence, conventional processing for electroless Ni--P deposition on such non-conductive substrates involves a sensitization pre-treatment with colloidal palladium (Pd) prior to immersion in the electroless Ni--P plating bath, e.g., as disclosed in U.S. Pat. Nos. 3,904,792; 3,961,109; 4,604,299; and 4,933,010, the entire disclosures of which are incorporated herein by reference. However, these sensitization pre-treatment procedures were developed mainly in the context of printed circuit board (PCB) manufacturing where microscopic (e.g., nanometer-dimensioned) defects in the produced Ni--P plating layers were not critical. Since the sensitizer, or activator, layer formed by such processing methodology consists essentially of discontinuous Pd, e.g., discrete Pd islands formed at spaced-apart locations on the substrate surface, it is therefore very difficult to obtain a fully sensitized surface (i.e., fully Pd-covered) required for obtaining nano-defect free Ni--P electrolessly plated layers thereon such as a required in the manufacture of high-density magnetic storage media. Additionally, the adhesion of Pd-activated electrolessly plated Ni--P coatings on such non-conductive substrate surfaces is obtained by acid etching and roughening to provide anchoring of the coating layer.
Disadvantageously, however, the chemical etching process frequently results in the formation of surface defects, e.g., cavities and pits.
4. Plating of metal-ceramics substrates: currently available metal-ceramics composites which are candidates for use as substrates in magnetic recording media comprise an Al or Al-alloy matrix and ceramic particles held within the matrix. Such metal-ceramics composites, e.g., Al-ceramics composites, are typically activated for Ni--P plating thereon by means of a zincating process. However, the inability to plate on the non-conductive ceramic particles within the conductive metal matrix results in the formation of discontinuous Zn films, i.e., spaced-apart islands, as with the wholly non-conductive glass, ceramic, and glass-ceramics substrates described above. Such discontinuous Zn film formation typically results in the formation of plating defects in the form of pits. As a consequence, the defect level of the plating layer(s) is essentially determined by the size and distribution of the ceramic particles within the metal matrix, and it is very difficult to achieve defect-free Ni--P seed layers for use in memory disks by the use of existing electroless process methodology.
Accordingly, there exists a need for an improved electroless plating process suitable for forming defect-free plating or seed layers required in the manufacture of high-density magnetic recording media utilizing alternative substrate materials, which process provides coatings which are adherent to the substrate as well as to layers formed thereon. In addition, there exists a need for an improved electroless processing methodology for manufacturing alternative substrate-based high-density magnetic recording media which is simple, costeffective, and fully compatible with the productivity and throughput requirements of automated manufacturing technology.
The present invention fully addresses and solves the above-described problems attendant upon the manufacture of high-density magnetic recording media and hard drives utilizing alternative-type substrates, while maintaining full compatibility with all mechanical aspects of conventional disk drive technology.